Obstructive sleep apnea/hypopnea syndrome (OSAHS) is a well recognized disorder which may affect as much as 1-5% of the adult population. OSAHS is one of the most common causes of excessive daytime somnolence. OSAHS is most frequent in obese males, and it is the single most frequent reason for referral to sleep disorder clinics.
OSAHS is associated with all conditions in which there is anatomic or functional narrowing of the patient's upper airway, and is characterized by an intermittent obstruction of the upper airway occurring during sleep. The obstruction results in a spectrum of respiratory disturbances ranging from the total absence of airflow (apnea) to significant obstruction with or without reduced airflow (hypopnea, episodes of elevated upper airway resistance and snoring), despite continued respiratory efforts. The morbidity of the syndrome arises from hypoxemia, hypercapnia, bradycardia and sleep disruption associated with the respiratory obstruction event and arousals from sleep.
The pathophysiology of OSAHS is not fully worked out. However, it is now well recognized that obstruction of the upper airway during sleep is in part due to the collapsible behavior of the supraglottic segment of the respiratory airway during the negative intraluminal pressure generated by inspiratory effort. Thus, the human upper airway during sleep behaves as a Starling resistor, which is defined by the property that the flow is limited to a fixed value irrespective of the driving (inspiratory) pressure. Partial or complete airway collapse can then occur associated with the loss of airway tone which is characteristic of the onset of sleep and which may be exaggerated in OSAHS.
Since 1981, positive airway pressure (PAP) applied by a tight fitting nasal mask worn during sleep has evolved as the most effective treatment for this disorder, and is now the standard of care. The availability of this non-invasive form of therapy has resulted in extensive publicity for sleep apnea/hypopnea and the appearance of large numbers of patients who previously may have avoided the medical establishment because of the fear of tracheostomy. Increasing the comfort of the system (e.g., by minimizing the applied nasal pressure) has been a major goal of research aimed at improving patient compliance with therapy.
PAP therapy has become the mainstay of treatment in Obstructive Sleep Disordered Breathing (OSDB), which includes Obstructive Sleep Apnea/Hypopnea, Upper Airway Resistance Syndrome, Snoring, exaggerations of sleep induced rises in collapsibility of the upper airway and all conditions in which inappropriate collapsing of a segment of the upper airway causes significant un-physiologic obstruction to airflow. This collapse generally occurs whenever pressure in the collapsible portion of the airway becomes sub-atmospheric (or more accurately lower than a “tissue pressure” in the surrounding wall at a critical location in the upper airway of the patient). PAP therapy is directed to maintaining pressure in the collapsible portion of the airway at or above the critical “tissue pressure” at all times. In conventional CPAP, this is achieved by raising the airway pressure in the entire respiratory system to a level higher than this critical pressure.
Conventional implementations of PAP therapies have either provided a single continuous pressure at the nose or a combination of such a continuous pressure with a lowering of pressure when the pressure is not thought to be needed (e.g., during expiration). Continuous PAP (“CPAP”) generally provides a constant pressure at least as large as the largest pressure necessary to prevent airway collapse. Some PAP therapies have provided and modified pressure profiles in an attempt to achieve the lowest (and presumably most comfortable) pressure which produces the desired therapeutic results.
For example, a procedure known as Bi-Level PAP is a known modification to CPAP. In Bi-Level PAP, a first constant pressure is set as an inspiratory pressure and a second lower constant pressure is set to be applied during expiration. The choice of the second pressure was originally based on the assumption that collapse of the upper airway occurs primarily during inspiration and that little or no collapsing force is generated absent the negative airway pressure generated during inspiration. However, Bi-level PAP has been shown to have little benefit in obstructive sleep apnea as the second expiratory pressure needs to be set at or near the level that would have been chosen for single pressure CPAP in order to prevent airway collapse. Bi-Level PAP is now generally restricted to patients who benefit from an unintended side effect of this mode of positive pressure (i.e., assisted ventilation that may arise when the difference between inspiration and expiration pressures are significant). However, as will be discussed below, patients who do not need assistance in breathing may find it difficult to draw the amount of air they desire as these systems react to their breathing by changing inhalation and exhalation pressures, and thus may not deliver the necessary pressure at the beginning of inspiration predictably.
Another modification of PAP is described, e.g., in U.S. Pat. No. 6,105,575. This PAP system provides to a patient a minimally sufficient pressure during at least a portion of a breathing cycle to perform at least one of the following functions at any given moment: (1) reduce cardiac preload and afterload and (2) prevent airway collapse. When operating to prevent airway collapse, the minimally sufficient pressure is determined by a summation of a pressure needed to prevent airway collapse and a pressure needed to overcome respiratory effort. This requirement to overcome respiratory pressure makes the system provide assisted ventilation, which, like Bi-Level PAP, is not need to treat obstructive sleep disordered breathing if there is no hypoventilation syndrome.